vkbench: Use nanoseconds in TimeTest

To make it comparable to glbench, use the same precision used in
GLBench.

This also changed how Run reports the metrics by average the last two
runs (same as glbench)

BUG=b:291637728
TEST=./vkbench --tests=copy.BufferStream --verbose

Change-Id: I8871598aa15f4a2bc3d7a9f3b578378ea57c4195
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vkbench/+/4935160
Reviewed-by: Ilja Friedel <ihf@chromium.org>
Tested-by: Ilja Friedel <ihf@chromium.org>
Commit-Queue: Ilja Friedel <ihf@chromium.org>
2 files changed